1. Field of the Invention
This invention relates generally to compact discs, and more particularly efficient integrated circuit processing in high speed compact disc drives.
2. Description of the Related Art
Compact disc drives have become increasingly popular due to their ability to rapidly access large quantities of data as well as provide fine quality digital play-back. To meet the need for increased data transfer rates, compact disc "CD" hardware engineers have been designing CD drives that are able to transfer data at speeds that are many times the rotational speed of normal audio CD audio (e.g., 4.times., 10.times., 24.times., . . . 50.times. . . . etc.). For example, when a CD contains normal audio, the processing and play-back of the audio data is performed at 1.times. speeds. Accordingly, CD drives must be capable of operating at various speeds in order to appropriately process the data contained on a CD media, whether the CD media contains pure audio data or some other type of stored data.
Although there are many types of CD drives that provide different rotational speeds depending on the type of CD media being read, a new type of CD drive, which is capable of maintaining a constant high rotational speed irrespective of what the CD media has stored therein has recently been developed. The various advantages of such CD drives are discussed in greater detail in a co-pending U.S. patent application entitled "An Improved Disc Drive", and having U.S. Ser. No. 08/917,792, which is incorporated by reference herein. Because the CD will now rotate at a constant high speed, the CD drive must be capable of reading and processing the data stored on the CD sufficiently fast to avoid introducing delays.
A common technique for increasing processing speeds has been to incorporate faster microprocessors, however, even fast processors have found reading and processing the CD media being spun at ever increasing speeds a challenge. By way of example, basic CD drive tasks, such as "seeking" to a location on the CD media in order to start play-back, are becoming increasingly difficult. In some cases, the microprocessor that is in charge of seeking to a particular "start" location on a CD media track has been found to be too slow to begin a play-back once the start location has been identified. That is, by the time the microprocessor determines that it has the correct start location, the CD media will have spun past its appropriate starting location. Many times, play-back may not start until a next sector is encountered.
FIGS. 1A through 1C illustrate, by way of background, techniques used to store audio data on a CD media 100. As is well known, the CD media 100 has a continuous track that spirals around the CD media 100, beginning at the inner region and ending at the outer edge. At the beginning of the track, a lead-in region typically contains a table of contents (TOC) that is used by the CD drive to ascertain where data recorded on the CD media 100 is located, in terms of minutes, seconds and frames (i.e., MSFs). As shown, the track of the CD media 100 is divided into many sectors 102, where each sector 102 contains 2352 bytes of data. The final sector 102 of the CD media 100 is then followed by a lead-out region, which signals the end of the CD media 100.
Besides the 2352 bytes of data, 98 bytes of subcode bytes are also included in each sector 102, such that there are 98 bits of P-subcode, 98 bits of Q-subcode, 98 bits of R-subcode, 98 bits of S-subcode, 98 bits of T-subcode, 98 bits of U-subcode, 98 bits of V-subcode, and 98 bits of W-subcode in each sector 102. As is well known, each of these subcode bits may be used for a number of identification purposes, however, only the 98 bits of Q-subcode are used to ascertain the absolute MSF of a particular sector 102. Of course, the Q-subcode is sometimes used for other processing and identification purposes as well.
FIG. 1B provides a closer examination of the typical contents of an audio sector that may be stored on an audio track. For example, each sector 102a-102n will typically contain a pre-gap region 104 that is typically used as a silent region. Each sector 102a-102n also contains 98 "eight-to-fourteen modulation" (EFM) frames 106 that have both audio data and subcode data. As shown in FIG. 1C, each EFM frame 106 typically contains a SYNC field 120, a subcode field 122, a data field 124, an ECC (C1) field 126, a data field 128 and an ECC (C2) field. As described above, a sector 102 has 98 bytes of subcode, and therefore, each EFM frame will contain 8 bits of subcode (i.e., a P-bit, a Q-bit, an R-bit, an S-bit, a T-bit, a U-bit, a V-bit, and a W-bit).
In operation, when a user wants to seek out to a particular MSF on the CD media, a head actuator (not shown) moves an optical reading head to the radial position where the desired data is believed to be located. To identify the location, the optical reading head is required to sequentially read out one Q-bit at a time from a sector 102 until all 98 Q-bits have been read. Once all 98 Q-bits are read, the CD drive must perform microprocessor operations to determine whether those 98 Q-bits define an MSF that is equal to the desired MSF. Once the microprocessor determines that the MSF values match, the CD drive must be quick enough to start a data transfer (i.e., begin reading for play-back).
As mentioned earlier, as disc speeds continue to increase, the microprocessors that are assigned the task of processing the 98 bytes of subcode for each sector 102, will find it challenging, if not impossible, to begin the data transfer before the next sector is encountered. Referring to FIG. 1B, after all 98 bytes of subcode for sector 102a have been read by the CD drive, and the microprocessor performs the necessary operations to determine that sector 102a has the correct MSF it was looking for (i.e., the found MSF), the microprocessor is required to initiate the "start" of a data transfer. However, even the fastest of microprocessors will experience processing latencies 140 and 142 in: (a) finding the correct MSF, and (b) triggering a start after the correct MSF is actually found.
Unfortunately, as disc speeds continue to increase, the pre-gap 104, which defines the time slot in which the microprocessor performs its identification and start processing will consequently decrease. When this happens, the CD drive will typically be unable to start a data transfer within the pre-gap area, and therefore start the data transfer somewhere within the sector, and therefore start inside one of the EFM frames 106. As a result, the CD drive may commence a song somewhere in the middle.
In view of the foregoing, there is a need for a compact disc drive having automatic start capabilities to ensure that a start is rapidly triggered when a seek to a particular MFS is requested.